With their outstanding thermal and electrical conductivities, silver platelets and silver flakes are an important class of materials, with applications in many industries, such as the electronics industry, to generate electrically conductive structures and to provide EMI shielding.
A variety of methods have been used to prepare silver flakes or silver platelets, such as vertical freezing, ball milling, epitaxial growth, gas evaporation, vacuum deposition, Langmuir-Blodgett films, and chemical precipitation. The silver flakes used in the electronic industry are almost exclusively produced by milling silver powders in various solvents in the presence of suitable lubricants (See, e.g., U.S. Pat. No. 4,859,241 to Grundy). The flattening of the silver particles results from mechanical forces (shear and impact) provided by the movement of the milling media, which usually contains 1-5 mm spheres of materials of different densities and compositions (glass, stainless steel, or ceramics). Because the majority of the silver powders used in the milling process contain large agglomerates of sub-micrometer or micrometer size particles, milling almost always leads to the formation of silver flakes with large average particle sizes (5-20 μm) and broad size distributions. Such materials are becoming less and less suitable for each succeeding generation of electronic devices, which require increasingly thinner and smaller conductive structures. Furthermore, friction- and impact-induced wear of the milling media leads to contamination of the resulting silver flakes, thereby reducing the product quality.
Precipitation-based silver flake production is a promising technology for meeting the demands of the electronic industry. For example, nano-size silver flakes with edges up to about 110 nm have been produced by reducing silver nitrate with hydrazine at the interface of a water/octylarnine bi-layer system (Yener et al., Langmuir (2002) 18:8692-99). However, such a system is not environmentally friendly, and would be complex and costly on a commercial scale. Silver platelets in the 30-120 nm size range were produced by irradiation of silver nanospheres (R. Jin et al., Nature (2003) 425:487-90), but this is a two-step process; furthermore, photochemical processes are rarely amenable to commercial-scale production.
There is a need in the art for silver flakes with widths in the range of 0.1 to 1 μm that can be produced economically on a commercial scale. Consequently, there is great interest in the development of new, cost-effective, and environmentally friendly protocols that are capable of producing uniform silver flakes on a commercial scale.